MACs are used in the integrity check process.
Providing a way to check the integrity of information transmitted over or stored in an unreliable medium is a prime necessity in the world of open computing and communications. Mechanisms that provide such integrity check based on a secret key are usually called "message authentication codes" (MAC). Typically, message authentication codes are used between two parties that share a secret key in order to validate information transmitted between these parties. (source)
There are 2 types of MACs:
- HMAC - "Keyed-Hashing for Message Authentication" uses hash functions
- CMAC - uses symmetric encryption algorithms
In both cases, the algorithm provides a way to validate integrity by adding passwords to check the authenticity.